Three-Phase Transformers -- Summary/Quiz

Summary

¦ Three-phase transformers are constructed by winding three separate trans
formers on the same core material.

¦ Single-phase transformers can be used as a three-phase transformer bank
by connecting their primary and secondary windings as either wyes or
deltas.

¦ When calculating three-phase transformer values, the rules for three-phase
circuits must be followed as well as the rules for transformers.

¦ Phase values of voltage and current must be used when calculating
the values associated with the transformer.

¦ The total power output of a three-phase transformer bank is the sum
of the rating of the three transformers.

¦ An open-delta connection can be made with the use of only two transformers.

¦ When an open-delta connection is used, the total output power is 86.6%
of the sum of the power rating of the two transformers.

¦ It is common practice to center-tap one of the transformers in a delta
connection to provide power for single-phase loads. When this is done,
the remaining phase connection becomes a high leg.

¦ The NEC requires that a high leg be identified by an orange wire or
by tagging.

¦ The center connection of a wye is often tapped to provide a neutral
conductor for three-phase loads. This produces a three-phase four-wire
system. Common voltages produced by this type of connection are 208/120 and 480/277.

¦ Transformers should not be connected as a wye-wye unless the incoming
powerline contains a neutral conductor.

2. Two transformers rated at 100 kVA each are connected in an open-delta
connection. What is the total output power that can be supplied by this
bank?

3. How does the NEC specify that the high leg of a four-wire delta connection
be marked?

4. An open-delta three-phase transformer system has one transformer center-tapped
to provide a neutral for single-phase voltages. If the voltage from line
to center tap is 277 V, what is the high-leg voltage?

5. If a single-phase load is connected across the two line conductors and neutral
of the transformer in Question 4 and one line has a current of 80 A and the
other line has a current of 68A, how much current is flowing in the neutral
conductor?

6. A three-phase transformer connection has a delta-connected secondary, and one
of the transformers has been center-tapped to form a neutral conductor.
The phase-to-neutral value of the center-tapped secondary winding is 120
V. If the high leg is connected to a single-phase load, how much voltage
will be applied to that load?

7. A three-phase transformer connection has a delta-connected primary and a
wye-connected secondary. The center tap of the wye is used as a neutral
conductor. If the line-to-line voltage is 480 V, what is the voltage between
any one phase conductor and the neutral conductor?

8. A three-phase transformer bank has the secondary connected in a wye
configuration. The center tap is used as a neutral conductor. If the voltage
across any phase conductor and neutral is 120 V, how much voltage would
be applied to a three-phase load connected to the secondary of this transformer
bank?

9. A three-phase transformer bank has the primary and secondary windings
connected in a wye configuration. The secondary center tap is being used
as a neutral to supply single-phase loads. Will connecting the center-tap
connection of the secondary to the center-tap connection of the primary
permit the secondary voltage to stay in balance when a single-phase load
is added to the secondary?

10. Referring to the transformer connection in Question 9, if the center
tap of the primary is connected to a neutral conductor on the incoming
power, will it permit the secondary voltages to be balanced when single-phase
loads are added?

11. What is the frequency of the second harmonic?

12. Which of the following are considered triplen harmonics: 3rd, 6th,
9th, 12th, 15th, and 18th?

13. Would a positive-rotating harmonic or a negative-rotating harmonic
be more harmful to an induction motor? Explain your answer.

14. What instrument should be used to determine what harmonics are present
in a power system?

15. A 22.5-kVA single-phase transformer is tested with a true-RMS ammeter and an
ammeter that indicates the peak value. The true-RMS reading is 94 A. The
peak reading is 204 A. Should this transformer be derated? If so, by how
much?

ON THE JOB… You are working in an industrial plant. A three-phase transformer
bank is connected wye-delta. The primary voltage is 12,470 V, and the
secondary voltage is 480 V. The total capacity of the transformer bank
is 450 kVA. One of the three transformers that form the three-phase bank
develops a shorted primary winding and becomes unusable. A suggestion
is made to reconnect the bank for operation as an open-delta. Can the two
remaining transformers be connected open-delta? Explain your answer as
to why they can or why they cannot be connected as an open-delta.

If they can be reconnected open-delta, what would be the output capacity
of the two remaining transformers?

ON THE JOB… You are a journeyman electrician working in an industrial
plant. You are to install transformers that are to be connected in open-delta.
Transformer A must sup ply its share of the three-phase load. Transformer
B is to be center-tapped so it can provide power to single-phase loads
as well as its share of the three-phase load. The total connected three-phase
load is to be 40 kVA, and the total connected single phase load is to
be 60 kVA. The transformers are to have a capacity 115% greater than the
rated load. What is the minimum kVA rating of each transformer?